This invention generally relates to a method and apparatus for providing a secondary braking force for an electronic vehicle brake system in response to a failure of a front electronic brake system.
An electronic braking system (EBS) includes an electronic control unit (ECU) that generates control signals, which are transmitted to front and rear vehicle brakes. Typically, the front brakes are on a front EBS circuit and the rear brakes are on a rear EBS circuit. In the event of a braking system failure, i.e. if one or both of the circuits fail, the system must be capable of achieving a specified level of braking performance. In other words, the EBS must be capable of providing sufficient secondary braking force if the primary system partially or completely fails.
Traditionally, if one of the circuits fails, it is necessary for the remaining circuit to provide the prescribed level of braking performance. However, this configuration is not always sufficient for certain types of vehicles. For example, tractor-trailers or other similar towing vehicles have lightly loaded rear drive axles when the vehicle is not towing anything. If the front EBS circuit fails under this condition, insufficient braking torque is applied to the rear drive axles before wheel lock-up occurs. This means that the prescribed deceleration cannot always be achieved.
One solution to this problem has been to utilize spring actuators on the front brakes to achieve the desired braking performance. If the front EBS circuit fails, the front brakes are braked by operation of the parking hand control and the rear brakes are braked by the rear EBS circuit. Disadvantages with this solution are that the spring actuators increase cost, add weight, and make the system more complex. Further, this solution is incompatible with vehicle braking systems that include an electronic park braking system.
Thus, it is desirable to have an electronic braking system for a vehicle that provides sufficient secondary braking in response to a front EBS circuit failure without requiring the use of mechanical spring actuators, and which is fully compatible with electronic park brake systems, as well as overcoming the other above mentioned deficiencies with the prior art.